Highly purified C3 and C5 were studied by polyacrylamide gel electrophoresis in the completely reduced and dissociated state. Both proteins contain one alpha and one beta chain. (Molecular weight equals 107000 and 70000 respectively in each protein). Degradation studies with trypsin revealed that the alpha chain of both proteins are the origin of anaphylatoxin-chemotactic factor (C3a and C5a respectively). It was also shown that C5a generated by trypsin and by C423 is not identical. Prolonged degradation with trypsin affected the alpha as well as beta chain of C3 but only the alpha chain of C5.